The present invention relates to an ionization apparatus capable of efficient ionization by utilizing a sputtering process carried out by high density plasma. The present invention also relates to a thin film forming apparatus capable of forming various thin films at a high rate and a high degree of efficiency with ions and neutral particles produced by the ionization apparatus, and to an ion source with a high degree of efficiency and a high degree of yield (high ion current) from which the ions produced by the ionization apparatus are extracted so as to form various thin films and so as to carry out an etching process.
So-called sputtering apparatuses for sputtering targets as such thin film formation elements in plasma, thereby forming thin films, have been widely used to form films of various materials in various fields. For instance, as shown in FIG. 1, a conventional sputtering apparatus in which a target 1 and a substrate 2 are disposed in opposing relationship with each other in a vacuum chamber 4, which is generally called a two-electrode sputtering apparatus (refer to F.M.D'HEURLE: Metall. Trans. Vol. 1, March, 1970, 725732), has been used and is well known to those skilled in the art. So is the three-electrode sputtering apparatus shown in FIG. 2, in which a third electrode 3 for emitting ions is additionally provided (W.W.Y. Lee and D.Oblas: J. Appl. Phys. Vol. 46, No. 4, 1975, 1728-1732). Furthermore the magnetron sputtering process (refer to R. K. Waits: J. Vac. Sci. Technol, Vol. 15, No. 2, 1978, pp. 179-187) in which, as shown in FIG. 3, a magnet 5 is used to apply a magnetic field of suitable strength to the target 1 so that high density and low temperature plasma is produced, whereby a thin film is formed at a high rate, has been used and is well known to those skilled in the art. These apparatus mainly comprise the vacuum chamber 4 with the target 1 as a thin film forming element and the substrate 2 upon which a thin film is formed, a gas introduction system, and a gas discharge system, and plasma is generated within the vacuum chamber 4.
In order to form a thin film at a high rate using one of the above-mentioned apparatuses, it is necessary to maintain the plasma at a high density. However, in the case of the two-electrode or diode sputtering apparatus, the higher the density of plasma, the more quickly a voltage applied to the target is increased. At the same time, the temperature of the substrate quickly rises because of the bombardment of the substrate with high-energy particles and high-energy electrons within the plasma, so the risk of damage to the formed film increases. Therefore, this apparatus can be only used for special heat-resisting substrates, thin film materials and film compositions. In the case of the three-electrode or triode sputtering apparatus, the plasma density is increased because the electrons are supplied from the third electrode into the plasma, but just like the two-electrode or diode sputtering apparatus, if a thin film is to be formed at a high rate, the temperature of the substrate quickly rises. As a result, only a limited small number of thin film materials and substrates may be used.
On the other hand, in the case of the high rate magnetron sputtering apparatus, the gamma (.tau.) electrons (secondary electrons) emitted from the target, which are needed to ionize the gas in the plasma, are confined over the surface of the target by both the magnetic field and the electric field, so that a dense plasma can be produced at a low gas pressure. In practice, the high rate sputtering processes are carried out at a low gas pressure of the order of 10.sup.-3 Torr, so that they are widely used for forming various thin films at a high rate. However, in the case of the sputtering apparatus just-described above, because of the bombardment by the ions in the plasma (mainly Ar.sup.+ ions), the high-energy neutral particles emitted from the target (mainly Ar reflected at the surface of the target), and the negative ions on the film being formed, changes in the composition of the thin film occur and the film or the substrate is damaged. In practice, it is well known to those skilled in the art that in the case of the formation of a ZnO film, the composition of the portion of the ZnO film immediately above the erosion area of the target and that of the other portion of the ZnO film are completely different from each other. Therefore, the bombardment of the substrate by the high-energy particles presents serious problems. In addition, the erosion area of the target is distributed locally, so that this apparatus has a low degree of utilization efficiency and cannot be used on an industrial scale.
Furthermore, when thin films are formed using the prior art sputtering apparatuses, gases and particles in the plasma are not ionized satisfactorily and particles which are sputtered to form a thin film land on the substrate in an almost neutral state. Therefore, since a sufficient degree of reaction activity cannot be attained, in order to obtain some oxides and thermally nonequilibrium materials, the temperature of the substrate must be raised to a high temperature ranging from 500.degree. C. to 800.degree. C. Moreover, almost all the power supplied to the plasma is consumed in the form of thermal energy, so that the ratio of the power used for the formation of the plasma (for electrolytic dissociation) to the power supplied to the apparatus is low. Consequently the power efficiency drops.
Furthermore, in any of the above-mentioned sputtering apparatuses, a stable discharge cannot be ensured at a gas pressure lower than 10.sup.-3 Torr, resulting in the defect that many impurities are entrapped in the thin film.
Meanwhile, Matsuo et al. have proposed a process for forming a thin film in which a plasma which includes a material to be deposited is generated under the electron cyclotron resonance (ECR) condition by using microwaves, and the plasma is drawn onto a specimen for forming the thin film (U.S. Pat. No. 4,401,054). However, this process cannot form thin metal films and thin metal compound films. Matsuo and Ono (U.S. Pat. No. 4,492,620) and Ono et al. (Jpn. J. Appl. Phys. Vol. 23, No. 8, 1984, L534-L536) have disclosed a microwave plasma deposition apparatus in which a target is sputtered by microwave plasma utilizing ECR and sputtered particles are deposited on the surface of the substrate, thereby forming a thin film.
These apparatuses accomplish sputtering by utilizing the excellent features of the microwave plasma, such as the capability of discharging at 10.sup.-5 -10.sup.-4 Torr and of producing highly active plasma. Therefore, these processes are excellent for accomplishing highly active sputtering at low gas pressures.
However, the sputtering rate is not very high and the ionization ratio of the particles sputtered from the target is low. Further the energy cannot be controlled in a satisfactory manner because the target is located outside of the plasma generating chamber.
Japanese Patent Application Laid-Open Nos. 61-87869 and 61-194174 disclose apparatuses in which microwave plasma generated under the ECR condition is confined by a mirror magnetic field so that sputtering can be accomplished by high density plasma. In these apparatuses, it is possible to sputter in a high vacuum of from 10.sup.-4 to 10.sup.-5 Torr. However, in the case of the former apparatus (No. 61-87869), since both the target and the substrate are disposed in high density plasma, there arises the problem that high-energy neutral particles and charged particles directly damage the surface of the substrate, resulting in the degradation of the formed thin film. It has the further problem that a mechanism for cooling the surface of the substrate becomes complicated because the temperature rises due to the high density plasma. In the Case of the latter (No. 61-194174) apparatus, the sputtering process is carried out by drawing plasma, which is generated in a mirror magnetic field, by another magnetic apparatus to the surface of the target. In this case, the target is located outside of the plasma generating chamber for generating high density plasma, so that the ionization ratio of the particles sputtered from the target is low and consequently this apparatus is not adapted to forming a thin film by highly reactive ions.
When thin films are formed by sputtering, the following conditions must be satisfied:
(1) A rapid temperature rise should not occur, since this would damage the substrate and the film, and the thin film should be formed at a high rate (that is, high density plasma is required).
(2) The energy of the particles should be controllable over a wide range.
(3) The dispersion of the energy of the particles should be reduced to as small a value as possible.
(4) The ionization ratio of the plasma must be high and the plasma must be active.
(5) Plasma should be generated at a low gas pressure.
So far, a thin film formation apparatus which can satisfy all of the conditions described above has not been realized.
Turning next to ionization apparatuses, such apparatuses may be used as ion sources. Various ion sources utilizing plasma have been widely employed, such as the so-called ion beam sputtering apparatus in which ions are sputtered onto a target, thereby forming thin films, and such as an etching apparatus which is used in the fabrication of integrated circuits. There are many types of ionization apparatuses, such as the Kaufman type, the duoplasmatron type, and so on. Of all the types, the Kaufman type has been most widely used as an ion source. In a Kaufman type ionization apparatus, as shown in FIG. 4, a filament 7 for emitting thermal electrons is mounted in a plasma generating chamber 6 and is used as a negative electrode. The discharge occurs in a magnetic field produced by an electromagnet 8, thereby generating plasma 9. The ions in the plasma are used to produce an ion beam 11 through a plurality of extraction grids 10. Refer to H. R. Kaufman et al.: J. Vac. Sci. Technol., Vol. 21, No. 3, 1982, pp. 725-736. In the Kaufman type ion source, the kinds of gases introduced into the plasma generating chamber 6 to generate ions are limited to inert gases such as Ar, because the filament 7 for emitting thermal electrons is used as an ion source. If a reactive gas were used, it would react with the filament 7, so that stable generation of plasma and extraction of ions would be possible. The apparatus has the further problems that the characteristics drop due to the ageing of the filament 7, maintenance of the source (such as replacement of the filament 7) is cumbersome, and reproducibility is impaired due to variations in the distribution of ion extraction which are caused by the variations in installation of the filament 7. In addition, the filament for emitting the thermal electrons is always exposed to the plasma 9 and the high-energy ions in the plasma impinge on the filament 7. As a consequence, in the extracted particles the material of the filament 7, such as tungsten, is intermixed as an impurity. With the above-mentioned ion source, the ions which can be extracted are limited to the ions of inert gases as described above, and it follows therefore that it is essentially impossible to obtain ions of metals such as aluminum (Al), copper (Cu), iron (Fe) and so on. The same is true for the duoplasmatron type ion source (refer to M. E. Abdelaziz and A. M. Ghander: IEEE Trans. Nucl. Sci., June 1967, pp. 46-52).
When an ion source is used for forming films or etching, it is preferable that the current density of the extracted ion beam be as high as possible. However, in the case of a conventional ion source using a filament, the amount (that is, the number) of ions is dependent upon the amount of electrons emitted from the filament. For this reason it has been essentially impossible so far to make an ion source from which a large amount of ions can be extracted. Furthermore, in the cases of the conventional ion sources, a stable discharge cannot be maintained in the plasma generating chamber at a gas pressure lower than 10.sup.-3 Torr. Therefore, the extracted ions contain a greater proportion of impurities.
U.S. Pat. No. 4,450,031 discloses an apparatus in which, in order to obtain a stable ion beam at a high current, ions are extracted from plasma through an electrode system consisting of a screening electrode and an ion extraction electrode. The plasma is generated by electron cyclotron resonance (ECR) excited by microwaves or by some other method. Japanese Patent Application Laid-Open No. 60-264032 discloses an apparatus in which the ion extraction electrode for extracting the ions from plasma generated by the microwave ECR process is improved. In these ion sources, a high current ion beam can be derived from high density plasma in a stable manner, but the kinds of ions obtained are limited to plasma generation gases, so that it is impossible to obtain ions except gaseous ions. That is, it is impossible to obtain a beam of metal ions.
The conditions required for an ion source may be summarized as follows:
(1) A great amount (i.e., number) of ions should be extracted (high current ions).
(2) Impurities should be excluded as much as possible.
(3) The energy of the ions should be controllable over a wide range.
(4) Not only the ions of inert gases, but also various other ions such as metallic ions, should be available.
However, not even one ion source which can satisfy all of the above-described conditions has been realized yet.